Already known, for example from document EP-0 793 120, is an adaptive mirror comprising:                a plurality of bar-shaped piezoelectric actuators joined to a rigid base by one of their ends and provided with electrodes by virtue of which electrical voltages can be applied to said piezoelectric actuators to generate electric fields therein; and        a flexible reflecting surface, borne by the other of the ends of said piezoelectric actuators and able to be locally deformed by each of these, the length of which varies under the action of said electric fields.        
Furthermore, it is known that variation in the length of a block of piezoelectric material parallel to the direction of the electric field applied to it depends on the electrical voltage generating said field, but is independent of this length.
Therefore, in the known adaptive mirror recalled hereinabove, in order to obtain a sufficient stroke for the end of each piezoelectric bar linked to said flexible reflecting surface to be deformed, it is necessary for said bar to consist of a stack of a plurality of piezoelectric elements, each of which is provided with a pair of electrodes, such that the stroke of said bar can be equal to the sum of the variations in thickness of said elements.
In this way, the larger the number of elements itself, the greater is the stroke of the composite piezoelectric bars. However, if the number of stacked elements is high, it is necessary for said stacked elements to have a large area to ensure a foundation sufficient for said composite bars. It is therefore impossible to obtain small pitch of said piezoelectric bars on said rigid base.
In these known adaptive mirrors a compromise must therefore be respected between the stroke and the pitch desired for the bars. In practice the minimum pitch that can be obtained is of the order of 3 mm for a stroke of the order of several micrometers.
However, numerous applications of adaptive optics require pitches of the piezoelectric bars of the order of 1 mm, again for a stroke of the order of several micrometers.